SaveAsText documentation doesn't address scope of separator argument

`SaveAsText` in the `Data` catalog has a `separator` parameter, which is specified as a `char`. However, this throws an `ArgumentOutOfRangeException` if the `separator` is not a "space, tab, comma, semicolon, or bar".

Currently, the only way to discover this delimiter limitation is to try and fail.

We have a few options. We could allow any character to be used as a delimiter, we could add documentation to the parameter that says it must be one of these characters, or, if we really do want to scope to a limited set of characters, then we could add a compile-time check by changing this to an `enum`.